1. Field of the Invention
This invention relates to the cleaning of well bores prior to cementing operations, and it more particularly relates to cleaning of well bores to improve cement bonding to formations and tubular members at the pay zone.
2. Description of Prior Art
It is common practice to clean a well bore at the pay zone before cementing operations to insure good bonding between the cement, well tubular members and formation. Present cement flushes remove drilling fluid and mud filter cake to insure clean well bores before introduction of the aqueous fluid cement. For example, a flush fluid can be used as a spacer before the cement so as to displace the mud materials. These spacers are used in relatively large amounts to insure cleaning of the well bore. However, water in the flush fluid and/or the cement slurry being used, cause viscosity and mobility changes in the mud residing in the well bore. As a result, channeling of the flush, and following cement, through the mud residues cause poor bonding of the cement to the formation and tubular members. Where the mud residues are from oil based mud, these problems are greatly increased.
The presence of water in the flush causes other severe problems. In pay zone formations sensitive to swelling (e.g. shales), presence of substantial water in the flush can (1) remove all mud filter cake from the well bore, (2) prevent proper cement bonding to the formation and (3) greatly reduced the permability of swelling formations. Unfortunately, cement slurries adversely effect residual mud materials and usually they become thickened with corresponding viscosity increases. Usually, these mud residue properties are caused by changes in the alkalinity drawn from the cement slurries. The thickened mud residues are less easily removed, and therefore more mud residue remains to degrade the cement bonding result. When oil based muds are used in the well bore, these undesired results are even more pronounced.